This invention relates to a wet process for preparing fine, plate-shaped particles of a ferrite having the magnetoplumbite structure.
Recently, in the field of composite magnetic materials, such as plastic magnets in sheet form, for example, there has been an increasing demand for fine and plate-shaped particles of ferrites. Also in the field of magnetic recording media, preference is often given to plate-shaped ferrite particles over needle-shaped particles as the development of new recording methods continues and the areas of application of the recording media expand.
Ferrites having the magnetoplumbite structure are known as suitable for such uses. Typical ferrite compounds of this class are Ba-ferrite, Sr-ferrite and Pb-ferrite, and it is also known that these ferrite compounds form useful solid solution with one another. General requirements for the plate-shaped ferrite particles for the aforementioned uses include the following items: (1) to be completely ferritized; (2) to be narrow in particle size distribution so as to easily and uniformly be dispersed in liquid or plastic materials; (3) to have a truly plate-like particle shape with very small thickness-to-width ratio; (4) to be so small in particle size that each ferrite particle in smaller than or nearly equivalent to a single magnetic domain; and (5) to be free of mechanical distortion.
Both dry processes and wet processes are known for the preparation of fine particles of ferrites having the magnetoplumbite structure, but in our view a wet process is more suitable than a dry process for preparing particulate ferrites of this class with success in realzing favorable properties including the above listed items.
As to the wet processes for the preparation of particulate ferrites of the magnetoplumbite structure, the most popular example will be the hydrothermal synthesis of Ba-ferrite described in Japanese Patent Specification Publication Nos. 46(1971)-3545 and 47(1972)-25796. In this process, either a salt of trivalent iron or goethite .alpha.-FeO.OH and a barium salt are employed as the principal materials, and, with the addition of a strong alkali as a neutralizing agent, these materials are subjected to hydrothermal treatment in an autoclave to obtain a Ba-ferrite in the form of the fine particles dispersed in an alkaline solution. Thus, it is characteristic of these processes to use either a salt of trivalent iron or an iron oxyhydroxide as the source of iron in Ba-ferrite.
From an industrial point of view, however, these conventional wet processes are disadvantageous in that they consume large quantitites of alkali, which is a relatively expensive material, in that they produce relatively large amounts of unwanted salts as a by-product and in that they need to carry out elaborate washing of the product thereby consuming of large quantities of water.